This invention relates to quickly curable unsaturated polyester compositions capable of providing superior surface quality, superior in stability in storage, and hence useful as automotive exterior body panels and other exterior parts. This invention also relates to sheet-like and bulk-like molding compounds obtained by impregnating such unsaturated polyester compositions in glass fibers, and molded products obtained by curing such molding materials.
Fiber-reinforced plastics (FRP) based on unsaturated polyester resins have superior characteristics not only regarding mechanical strength and resistance against heat, water and chemicals but also regarding productivity, and are being used extensively in bathtubs, water tank panels and bathroom sinks. Recently, unsaturated polyester sheet molding compounds (SMC) and bulk molding compounds (BMC) have come to be recognized as plastic materials for automotive exterior body panels and are being applied not only to main exterior panels such as engine hoods, roofs and trunk lids, but also to other exterior parts such as spoilers, air intakes and rocker panels.
The SMC and BMC to be used for automotive exterior body panels are required in particular to have superior surface quality, fast curing property and high productivity. If the curing speed of SMC and BMC can be improved even by seconds, for example, this alone can improve productivity significantly. For this reason, there have been various proposals for quick curing catalysts. (See, for example, U.S. Pat. No. 4,278,558.) If the curability of SMC or BMC is increased, however, its surface smoothness and stability in storage are generally adversely affected. It has been known to use thermoplastic resin such as polymethyl methacrylate, polyvinyl acetate and styrene butadiene block copolymers as a low profile additive, and it has been reported that the surface smoothness can be improved, when the curing speed has been increased, by using alkane polyol polyacrylate or alkane polyol polymethacrylate (Japanese Patent Publication Tokkai 01-315458). Stability in storage, on the other hand, is significantly affected if the curing speed is increased. Stability in storage may be improved if a large amount of polymerization inhibitor such as PBQ (parabenzoquinone) is added, but the start of curing reaction is significantly delayed, and not only is the purpose of improving the curing speed defeated, but the surface smoothness is also significantly affected. In summary, it has been a serious problem to secure stability in storage in the development of molding materials having both quick curing property and superior surface quality.